On the input common-mode voltage range of CMOS bulk-driven input stages

In this paper the response of a bulk-driven MOS Metal-Oxide-Semiconductor input stage over the input common-mode voltage range is discussed and experimentally evaluated. In particular, the behaviour of the effective input transconductance and the input current is studied for different gate bias voltages of the input transistors. A comparison between simulated and measured results, in standard 0.35-µm CMOS Complementary Metal-Oxide-Semiconductor technology, demonstrates that the model of the MOS transistors is not sufficiently accurate for devices operating under forward bias conditions of their source-bulk pn junction. Therefore, the fabrication and the experimental evaluation of any solution based on this approach are highly recommended. A technique to automatically control the gate bias voltage of a bulk-driven differential pair is proposed to optimize the design tradeoff between the effective input transconductance and the input current. The proposed input stage was integrated as a standalone block and was also included in a 1.5-V second-order operational transconductance amplifier (OTA)-C lowpass filter. Experimental results validate the effectiveness of the approach